1. Field of the Invention
This invention relates to enhancing the water affinity of a wide variety of membranes coated with extremely hydrophilic polymers and specifically to membranes coated with poly(2-acrylamido-2-methyl-propane sulfonic acid) (PAMPS). It particularly relates to use of an oxygen plasma for such enhancement.
2. Review of the Prior Art
Ultrafiltration and/or dialysis membranes are barriers which permit selective transport of solvent and some solutes across them. They are used in a variety of industrial applications, ranging from the re-concentration of dilute paint dispersions to the isolation of food products and pharmaceuticals, and in biomedical applications such as hemodialysis. Such membranes are typically produced in three general physical formats: sheet membranes, hollow fibers, and tubes.
Most permselective membranes disclosed in the literature are produced by solution/coagulation processes wherein a solubilized polymer is cast as a thin film and then coagulated in a non-solvent liquid. The non-solvent is generally selected from one of many liquids which are miscible with the solvent but which reduce its solvent power, causing loss of polymer solubility when added to the solvent.
Conventional synthetic membranes used for reverse osmosis (RO) and for ultrafiltration (UF) are made of polymers that are insoluble in the fluids, such as water and air, that act on the membranes. Typically, a linear polyamide, polysulfone, or cellulose acetate is cast from an organic solvent and coagulated in water. Although such membranes are rigid and physically strong, they are hydrophobic in nature and tend to foul through adsorption of hydrophobic particles and solutes in the feed stream. Such fouling is a major problem in industrial use of membranes, making frequent cleaning or costly pretreatment necessary.
Although hydrophilic polymers, particularly highly sulfonated ones, have been shown to resist such adsorptive fouling, they either dissolve in water or form a soft gel. Useful membranes may be formed of such polymers only by a high degree of crosslinking, so that they swell not more than 3-5 times by weight when soaked in water. Since the pore structure of such membranes is generally created by coagulation and since monomers are essentially incoagulable, the only practical route is post-crosslinking of the linear polymer so that the individual polymer chains are joined at many points to yield an interconnected network. The crosslinks may be ionic in character, as in complexes of poly-acids with poly-bases, but covalent or chemical crosslinking is stronger, more resistant to hydrolysis, and more versatile in its applications.
Since crosslinked polymers cannot be dissolved, melted, or cast, the polymerization or post-crosslinking must be carried out in the final physical shape required. The most common way of achieving covalent crosslinking is to use a polyfunctional monomer in the polymerization reaction itself. Problems, however, often arise in the preparation of films and membranes when catalysts must be introduced or inhibitors such as oxygen must be excluded from the polymerization reaction. Furthermore, the monomers themselves are often too toxic, volatile, or fluid to be conveniently processed in this manner. All of these factors make the manufacturing process difficult and costly.
Conventional methods of preparing hydrophilic polymer membranes involve use of a solution of the polymer in an appropriate solvent such as ethanol or water at a concentration ranging from 2 to 20% polymer. A polymeric membrane is employed as a substrate for depositing the polymer thereon as a thin coating.
For example, as taught in U.S. Pat. No. 5,039,420, highly hydrophilic, substantially uncharged semipermeable membranes based on copolymers of acrylonitrile and hydroxy-C.sub.2 -C.sub.4- alkyl esters of (meth)acrylic acid, preferably in flat sheet or hollow fiber form, are prepared by dissolving the AN copolymers in a polar organic solvent in which they are very soluble and then by casting the copolymer solution on a suitable substrate or by extruding to produce hollow fibers, and finally by quenching with a coagulation solution.
Polymer membranes traditionally are made hydrophilic by incorporation of groups such as hydroxyl, carboxyl, carboxy salts, or sulfonic acid. Conventional RO membranes tend to suffer compaction or swelling over a period of time and are highly susceptible to biological and chemical attack (particularly chlorination).
For certain filter applications, as for example for a spinning microfilter employed to separate smoke particles from air, use of a membrane which is extremely hydrophilic is essential to prevent clogging of the filter. Increases in hydrophilicity increasingly prevent clogging and enhance filter efficiency. Clogging is prevented because deposited hydrophobic material is scrubbed off by the Taylor vortices generated by the spinning of the inner cylindrical filter.
When polymeric gels or films are prepared, as taught in U.S. Pat. No. 4,596,858, by dissolving in a suitable solvent a linear polymer or polymers containing pendant amidocarbonyl or oxycarbonyl groups, hydroxyl groups (present as either pendant groups from the polymer or as a low molecular weight polyol), and a strongly acidic catalyst (which may also be a pendant group from the polymer), by drying or coagulation of the gel to remove the solvent, and by curing the gel at a high temperature, some of the amido-carbonyl or oxycarbonyl groups are alcoholized to form ester linkages that crosslink and insolubilize the gel. These post-crosslinked membranes have physical strength and controlled porosity and mitigate the effect of polymer chain scission, thus enabling reverse osmosis membranes, for example, to keep their selectivity under conditions of use.
Crosslinked poly(acrylamido-2-methylpropane sulfonic acid) (PAMPS) membranes which have been prepared as taught in Examples 7 and 22 of U.S. Pat. No. 4,596,858 have been found to be more hydrophilic than similar materials which have a carboxyl or aliphatic group in place of the sulfonic acid group. Such carbonyl-containing PAMPS membranes, films, and solid gels are stated to have excellent strength, controlled pore size, and controlled swelling.
U.S. Pat. No. 5,028,453 describes a procedure for reducing the tendency of a surface to foul, by treatment of any of a variety of surfaces with plasma of a compound which generates hydroxyl groups. Generating compounds include a diol, a triol, or a polyol containing four or less carbon atoms per molecule. Surfaces for which hydrophilicity is increased include glass, ceramics, alumina, steel, carbon, and a wide variety of polymer materials.
Applicant has found experimentally that sufficiently cross-linked PAMPS membranes can be used to separate oil/water mixtures. However, the processes disclosed in the prior art for producing cross-linked PAMPS do not produce sufficient cross-linking for oil/water separation.
There is accordingly a need for a method of additional solid-state crosslinking of PAMPS membranes.
Other applications of oxygen plasma treatments include treatment of ultrahigh molecular weight polyethylene fibers. Such treatment has been shown by Occhiello et al, in J. of Appl. Polym. Sci. 42, 551-559 (1991), to enhance adhesion to a resin matrix, and adhesion can be related to surface free energy. These investigators showed that treatment increases surface concentration of --OH, --C.dbd.O, and --COOH groups. Treatments were done for as long as 10 minutes; however, no increase in surface free energy was found after 2 minutes when 70-100 watts of power were used at a pressure of 0.1-0.2 Torr.
Oxygen plasmas have also been employed by Ramesh et al, Polym Sci: Part B. Polym. Phys. 29, 1031-1034 (1991), to etch polymer mixtures of aliphatic and aromatic polymers for revealing their microstructures. Aliphatic polymers are more susceptible to being etched away than are aromatic polymers, and the plasma etch therefore preferentially eats away the aliphatic component, thereby exposing details of the microstructure which are observable with a scanning electron microscope. For this work, the polymer mixture was subjected to an oxygen plasma for 10 minutes at 50 watts with an oxygen pressure of 400 mTorrs.